Psychophysics of reading and illumination needs during reading in visually impaired subjects with age-related macular degeneration
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- Institutt for fysikk 
The primary aim of this study was to contribute to a better understanding of how we can optimize reading conditions for subjects with age-related macular degeneration (AMD). In order to achieve this goal, we have developed a new test, The Tambartun Oral Reading Test which was used as an assessment tool during the reading experiments. All subjects were tested with a number of psychophysical tests, such as different types of visual acuity tests and contrast sensitivity tests, and the results were correlated with reading performance. For each subject, we measured acuity, reading speed, and reading comfort as a function of luminance. The AMD-subjects underwent thorough medical and optometric examinations. Initially, 22 subjects with AMD were referred to the study. Only 13 subjects, classified as visually impaired, met the strict selection criteria. The relatively low number of participants calls for caution when interpreting the data. However, since we restricted our study to patients with a “pure” AMD diagnosis, with no other ailments, we believe the data are likely to give a good indication of the variation one can expect to find within this patient group. For most subjects, letter visual acuity and grating visual acuity improved with increasing retinal illuminance, but not in proportion with one another. For AMD-subjects whose letter acuity was above or equal to 0.2 (minutes of arc)-1, the two acuity measures converged at relatively high light levels, while for AMD-subjects with letter visual acuity below 0.2, grating acuity was always considerably higher than letter acuity. This dichotomy demonstrates that grating acuity cannot be equated with letter acuity. This conclusion is of importance whenever acuity is used as a criterion to classify the visually impaired with regard to their legal and social status, for instance in non-communicative elderly subjects with AMD. Compared with a control group of normally sighted individuals, luminance contrast sensitivity was markedly reduced, particularly at the higher spatial frequencies. Unlike the control group, many of the AMD-subjects showed reduced binocular contrast summation or binocular inhibition for a narrow or extended frequency band. Among all visual functions tested, including letter acuity and grating acuity, luminance contrast sensitivity correlated best with reading speed. For most AMD-subjects, maximum contrast sensitivity - measured in terms of combined cone modulations - was about the same for achromatic gratings and isoluminant red-green gratings. The higher contrast sensitivity to red-green colour differences over achromatic differences, observed in subjects with normal vision, has somehow been lost in AMD-subjects. For all subjects it was possible to identify both the optimal light level, as well as a larger, but still satisfactory, range of light levels for reading. While the mean reading speed for the normally sighted age-matched control group was relatively constant at photopic light levels, the variation was much larger within the AMD group. All AMD-subjects achieved best reading performance for luminance levels above 80 cd/m2. Seven participants obtained maximum reading speeds for a luminance value of between 80 and 400 cd/m2. Five participants had a maximum reading rate for luminances that were equal to or higher than 300 cd/m2. The last of the 13 participant never achieved functional reading, since her maximum reading speed never exceeded 15–20 words per minute. For some of the AMD-subjects, the light levels at which maximum near letter visual acuity was obtained could serve as an indicator of lighting needs during reading. For others, this was a less reliable guide for determining lighting needs. Self reports of light preferences were thus needed as a supplement to objective measurements. Such evaluations should be included in test procedures. In conclusion, we have found that it is possible to optimize reading conditions for the AMDsubjects by charting individual needs and conducting thorough psychophysical measurements. Guidelines are proposed for assessments and practical rehabilitation measures, and recommendations are offered for future studies.